1. Field of the Invention
The present invention relates to a rear projection display apparatus and a screen unit that are able to project a magnified image from an image source onto a transmissive screen via a projection lens.
2. Description of the Related Art
A well-known type of rear projection display apparatus projects image light emitted from an image source formed of a CRT (Cathode Ray Tube) and an optical system using optical elements, such as a liquid crystal panel, onto a transmissive screen via a projection lens to form a magnified image thereon.
Since a screen unit in such a rear projection display apparatus is required to have various functions for projecting a good image, and includes in combination a plurality of screens having specific functions.
In general, the screen unit includes a Fresnel lens sheet located on the side of the image source, and a lenticular lens sheet located at the rear of the Fresnel lens sheet, as disclosed in, for example, Japanese Unexamined Patent Publication No. 6-6739.
FIG. 7 is a perspective view showing the structure of a screen unit 30 including a Fresnel lens sheet 31, a lenticular lens sheet 32, and the like.
Image light projected by a projection lens (not shown), which is placed after the aforesaid CRT or optical system, is gathered and directed toward a viewer by Fresnel lenses in the Fresnel lens sheet 31, which corrects variations in luminance from point to point on the viewed screen.
The lenticular lens sheet 32 placed behind the Fresnel lens sheet 31 contains a light diffusing agent therein. Image light is transmitted through the lenticular lens sheet 32 while being diffused by the light diffusing agent, thereby projecting an image. The lenticular lens sheet 32 also controls the light diffusion properties in the horizontal and vertical directions, and causes the light to converge by the lensing action of a plurality of lenticular lens elements located on the incident side.
Furthermore, external-light absorbing layers 32a are formed in vertical stripes at portions on the emergent side of the lenticular lens sheet 32 other than portions where the light converges so that they cover predetermined areas on the emergent side, thereby reducing the influence of external light from outside the display apparatus without blocking image light to be projected onto the screen, and improving the image contrast.
Although it is only necessary to increase the covering ratio of the external-light absorbing layers 32a in order to reduce the influence of external light and to improve the image contrast, if the covering ratio exceeds a predetermined value, image light is blocked by the external-light absorbing layers 32a, and is decreased in amount. Therefore, in a rear projection display apparatus using a CRT as an image source, the covering ratio of the external-light absorbing layers 32a is limited to, for example, about 40% according to the structure of the optical system and the like.
In a rear projection display apparatus using three CRTs corresponding to three colors R, G, and B, as shown in FIG. 8, image lights of three colors emitted from a red CRT 40R, a green CRT 40G, and a blue CRT 40B are magnified and projected onto a screen unit 30, which is composed of a Fresnel lens, a lenticular lens and the like, via projection lenses 41R, 41G, and 41B, respectively.
Since the CRTs 40R, 40G, and 40B are required to be placed so that their centers are opposed to about the center of the screen unit 30, as is shown, the CRT 40G is placed in front of the screen unit 30, and the CRTs 40R and 40B on both sides of the CRT 40G are placed at an angle of, for example, about 10.degree. with respect to the screen unit 30. Thereby the colored lights are incident on the screen unit 30 at different angles and are not parallel to each other in front of the screen unit 30, and therefore, the covering ratio is set at about 40% in consideration of these circumstances, as mentioned above.
In recent years, a rear projection display apparatus has also been known that uses as an image source an optical system composed of a liquid crystal panel and the like and that magnifies and projects an image formed by the optical system by a single projection lens.
FIG. 9 is a schematic view of the rear projection display apparatus in which an optical system 51 composed of three liquid crystal panels corresponding to colors R, G, and B and a screen unit 30 are placed in a housing 50.
An image formed by the optical system 51 located in the lower part of the housing 50 is magnified by a projection lens 52, reflected by a mirror 53 as shown by the arrows, and reaches the screen unit 30.
The configuration of the optical system 51 will now be described with reference to FIG. 10.
In a light source 55, a lamp 56 formed of, for example, a metal halide lamp, is placed at the focal point of a parabolic mirror, and light almost parallel to the optical axis of the parabolic mirror is emitted from an opening of the lamp 56. Unnecessary beams, in the infrared and ultraviolet regions, of the light emitted from the light source 55, are blocked by an UV-IR cutting filter 57, and only the effective beams are directed to a lens array section 58.
The lens array section 58 includes optical elements, such as a PBS (Polarizing Beam Splitter), and it polarizes P-polarized light and S-polarized light emitted from the light source 55 and outputs, for example, P-polarized light.
That is, the lens array section 58 allows the light from the light source 55 passed through the UV-IR cutting filter 57 to be polarized into P-polarized light and to be effectively and uniformly radiated onto effective apertures of liquid crystal panels 62, 63, and 68.
Between the lens array section 58 and the effective apertures of the liquid crystal panels 62, 63, and 68, dichroic mirrors 59 and 60 are placed to separate the light emitted from the light source 55 into lights of red, green, and blue light.
In this example shown in FIG. 10, the dichroic mirror 59 first reflects red light R, and transmits green light G and blue light B. The red light R reflected by the dichroic mirror 59 is deflected by 90.degree. by a mirror 61, and enters the red liquid crystal panel 62.
On the other hand, the green light G and the blue light B transmitted through the dichroic mirror 59 are separated by the dichroic mirror 60. That is, the green light G is reflected, deflected by 90.degree., and directed to the green liquid crystal panel 63. The blue light B passes through the dichroic mirror 60, travels straight, and is directed to the blue liquid crystal panel 68 via a relay lens 64, a mirror 65, a relay lens 66, and a mirror 67.
The colored light modulated by the liquid crystal panels 62, 63, and 68 is synthesized by a crossed dichroic prism 69 serving as a light synthesizing element. The crossed dichroic prism 69 is composed of a reflecting plane 69a and a reflecting plane 69b. The red light R and the blue light B are reflected toward the projection lens 52 by the reflecting plane 69a and the reflecting plane 69b, respectively, and the green light G is transmitted through the reflecting planes 69a and 69b. Therefore, the R, G, and B light is combined, and magnified and projected by the projection lens 52 onto the screen unit 30 shown in FIG. 9.
In such a rear projection display apparatus, since image light is magnified and projected by the single projection lens 52, it enters the screen unit 30 from one direction. Therefore, it is possible to set the covering ratio of the external-light absorbing layers 32a to, for example, about 80%, which is considerably higher than that of the aforesaid display apparatus using three CRTs shown in FIG. 8.
In order to maximize the covering ratio of the external-light absorbing layers 32a without blocking image light, as disclosed in, for example, Japanese Examined Patent Publication No. 7-19029, lens elements for constituting the lenticular lens sheet 32 are each shaped like a part of a longitudinal convex surface of an ellipsoid whose eccentricity is equal to the reciprocal of the refractive index of the lens medium, and each external-light absorbing layer 32a is formed at the focal point of the ellipsoid.
In the rear projection display apparatus shown in FIGS. 9 and 10, image light can be projected by the single projection lens 52 in one direction. Since the light beam emitted from the image source is intense and narrow, however, light diffused by the aforesaid light diffusing agent contained in the lenticular lens sheet 32 sometimes causes interference. This is a phenomenon called scintillation in which a part of the image projected on the screen shines brightly, which significantly reduces the quality of the image.
In order to lessen scintillation, a light diffusing structure for diffusing light may be placed at a position closer to the projection lens 52 than to the lenticular lens sheet 32, as disclosed in, for example, Japanese Unexamined Patent Publication No. 8-313865 and the like. This light diffusing structure is formed by mixing a light diffusing agent into the Fresnel lens sheet 31, or placing a separate light diffusing sheet before the Fresnel lens sheet 31.
Scintillation appears more pronouncedly as the light beam from the image source becomes narrower and intenser. Therefore, it is necessary to set the haze, which represents the light diffusion property of the light diffusing structure, according to the design of the optical system 51 affecting the light beam, such as the lamp output, the luminous characteristics, the sizes of the lens array and the liquid crystal panel and the f-number of the projection lens, the screen size of the screen unit 30, and the like. For example, the haze is set lower as the screen size of the screen unit 30 increases, and is set higher as the screen size decreases.
If the haze of the light diffusing structure is set lower than required, glare appears on the screen owing to scintillation. On the other hand, when the haze is set higher than required, light is excessively diffused, and the peak luminance of the image is impaired. Therefore, the haze is set within the range of, for example, about 60% to 80% according to the configuration of the optical system 51.
The haze is the proportion of the diffused transmittance to the total light transmittance that is measured by using an integrating sphere beam-transmittance measuring instrument according to Section 6.4 of the Japanese Industrial Standards K7105. For example, the haze of 60% means that 60% of light is diffused and emitted outwardly from the center.
For example, when a light diffusing structure with a haze ranging from 60% to 80% is used for the lenticular lens sheet 32 on which the external-light absorbing layers 32a are formed at a covering ratio of 80%, image light diffused by the light diffusing structure is blocked by the external-light absorbing layers 32a, and the amount of image light to be transmitted through the screen unit 30 is reduced.
That is, if the optimum covering ratio is not selected for the haze that is set according to the screen size of the screen unit 30 and the configuration of the optical system 51, the image obtained is not the same as the original one.